The invention relates to a process and an apparatus for separating polymers in solution from solids which are insoluble in the solution. The invention further relates to the separation of sulfur-containing polymers from undissolved solids.
In the preparation of polymers by polycondensation, solids are frequently formed as by-products, sometimes in a subsequent neutralization reaction. This applies, for example, to the preparation of polyphenylene sulfide (PPS) from sodium sulfide and p-dichlorobenzene in a polar solvent in accordance with the simplified reaction equation:
Na2S+C6H4Cl2xe2x86x92[xe2x80x94C6H4xe2x80x94Cxe2x80x94]+2NaCl
In known processes, the sodium chloride by-product which is insoluble in the reaction mixture is removed after the reaction by repeated washing with water and subsequent filtration of the polymer. However, these and related processes for other high-performance polymers such as polyaramides, poly(ether)sulfones or polyether ketones have the disadvantage that large amounts of salt-containing wastewater are obtained.
DD-A-241 368 describes an apparatus for the removal of water-soluble solids. Here, undissolved solid is separated from an epoxy resin solution by means of a combined process comprising sedimentation and dissolution of the solid in water. In operation, the apparatus disclosed has a phase boundary between the water-imiscible polymer solution at the top and the water as extractant for the solid underneath in the middle region. Separation processes using this apparatus consequently produce dilute, aqueous salt solutions.
EP-A 0 220 490 describes hot filtration or centrifugation at temperatures above 210xc2x0 C. for the separation of salt from PPS and a polar solvent. However, in industrial use, this method has the disadvantage that filtration apparatuses or centrifuges for this temperature range require costly constructions. At T greater than 210xc2x0 C. and the prevailing pressures, sealing moving, in particular rapidly rotating, components presents difficulties.
WO-A-96 11 968 describes the separation of the insoluble solids from dissolved polymers in order to recover filled polymers. In the method described there, the solids, for example glass fibers, are separated from the polymer solution by filtration. However, the known processes are capable of improvement since dilute washing liquors are obtained and the removal of the solid from the filter apparatuses is difficult, particularly at high temperatures under pressure.
It is an object of the present invention to provide a process and an apparatus by means of which solids are separated from dissolved polymers and the solid discharged has been substantially freed of polymers.
The object of the present invention is achieved by a process for separating a dissolved polymer from solids, wherein the solid is separated off by sedimentation and the sedimented solid is washed in countercurrent with a solvent.
It has been found that solids can be separated from dissolved polymers by sedimentation. According to the invention, a washing process simultaneously takes place in the sedimentation apparatus, by means of which the losses of polymer caused by adhesion of polymer solution to the solid can be minimized. The sedimentation and washing sequence can be repeated in a plurality of steps or stages until a prescribed residual content of polymer in the solution adherent to the solid is achieved.
The process of the invention can also be carried out simply at high temperatures and pressures. The separation process preferably occurs continuously without dilute washing liquors being obtained. The polymer concentration in the overflow from the sedimentation apparatus is greater than 50%, preferably greater than 70%, of the concentration at the inlet.
In the present context, the term polymer includes all macromolecular, predominantly organic compounds which dissolve in a solvent, if appropriate at elevated temperature under pressure.
The process of the invention is particularly suitable for polymers in whose preparation solids, for example salts, are formed as by-products and have to be separated off. The solids can be formed directly in the polycondensation process or in a subsequent neutralization of acids or bases. Examples of such processes are the preparation of polyaramides from diamines and diacid chlorides, of polycarbonates from diphenols or diphenoxides and phosgene, of polysulfones, polyether sulfones or polyether ketones from diphenoxides and dihalogenated aromatic hydrocarbons or the preparation of polyarylenesiloxanes from diaryidichlorosilanes and diphenoxides.
The process of the invention is of particular relevance for polymers which dissolve only at elevated temperature under pressure and in the case of which the removal of the solids therefore requires costly apparatus. Such polymers include, in particular, sulfur-containing polymers.
Sulfur-containing polymers are polymers comprising arylene sulfide units. The arylene constituents of the arylene sulfide units comprise monocyclic or polycyclic aromatics or linked aromatics. The aromatics can also contain heteroatoms. Examples of such aromatics, which may be substituted or unsubstituted, are benzene, pyridine, biphenyl, naphthalene, phenanthrene. Examples of substituents are C1-C6-alkyl, C1-C6-alkoxy, carboxyl, amino and sulfonic acid groups. Linked aromatics are, for example, biphenyl or aromatics joined by other bridges (arylene ethers).
Preferred sulfur-containing polymers are polyarylene sulfides, in particular polyphenylene sulfide.
Possible solids are all substances which, under the conditions under which the sedimentation apparatus is operated, are present in solid form and have a density different from the liquid mixture. Examples are salts, substances used as fillers, e.g. glass fibers, carbon fibers, titanium dioxide, gypsum or other solids.
Salts can be organic or inorganic, i.e. can consist of any combination of organic or inorganic cations with organic or inorganic anions. They have to be at least partially insoluble in the reaction medium and have a density different from that of the liquid reaction mixture. Typical representatives of the inorganic salts are the halides of alkali or alkaline earth metals which are frequently formed as by-product of a polycondensation. Typical representatives of organic salts are carboxylates of the alkali metals, of the alkaline earth metals, of ammonium or of organically substituted ammonium cations which are, according to the prior art, used as promoters in, for example, the preparation of sulfur-containing polymers. For the purposes of the present invention, carboxylates are the solids of aliphatic carboxylic acids, e.g. acetic acid or propionic acid, or aromatic carboxylic acids, for example benzoic acid, and also solids of polyfunctional carboxylic acids.
Apparatuses suitable for separating off the undissolved solids are ones which allow the solid which has been separated off to be washed with solvents. The separation is preferably carried out using an apparatus which makes it possible for the solid to be separated off to be washed in countercurrent in a number of stages and which can also be used at elevated pressures and temperatures and which allows effective washing of the solid to be separated off with small amounts of solvent, so that no highly dilute polymer solutions are formed. These objectives can be achieved by means of the sedimentation apparatuses described below.
The polymer concentration of the mixture to be separated is preferably up to 70% by weight. Particular preference is given to 10 -50% by weight. The polymer concentration in the liquid discharged together with the solid is from 0% to 70% of the concentration at the inlet, preferably from 0% to 10% and particularly preferably from 0% to 5%.
The sedimentation of the insoluble solid occurs at temperatures at which the polymers are liquid or in dissolved form. In the case of high-performance polymers which often dissolve only at elevated temperatures, the sedimentation apparatuses are operated at elevated temperatures and pressures. The temperatures are preferably in the range from 50xc2x0 C. to 300xc2x0 C. and the pressures are preferably in the range from 0 bar to 50 bar.
Materials of construction which are particularly suitable for the sedimentation apparatus are stainless steel, titanium or zirconium or other materials which are mechanically stable and are not subject to corrosive attack under the operating conditions.
The effectiveness of the solids removal by sedimentation depends on the particle size and particle size distribution of the undissolved solid, on the viscosity of the polymer solution and on the density difference between solid and liquid phase.
The mixture to be separated can be the reaction mixture of a polymerization which contains an undissolved solid. Since the viscosity increases with increasing conversion in the polymerization reaction and the particle size of the solid decreases during mixing of the reaction solution, it may be advantageous to carry out the removal of the solid by sedimentation before the final molecular weight has been reached. For this reason, a preferred embodiment of the invention provides for the solid to be separated from the reaction mixture before the desired molecular weight has been reached.
According to the invention, preference is given to a process for preparing polymers in a solvent, wherein
a) a polymer having a low molecular weight is prepared in solution,
b) the solid which is undissolved in the reaction mixture is separated off by sedimentation and washed and
c) the mixture which has been freed of the solid is polymerized further until a particular molecular weight has been reached.
The mean molar mass of the polymers, expressed by the weight average Mw, of the step a) is in the range from 500 g/mol to 30,000 g/mol, preferably from 1000 g/mol to 20,000 g/mol and particularly preferably from 2000 g/mol to 15,000 g/mol.
At the end of step a), the solid is separated off by sedimentation and, for example, washed with solvents. The separation is carried out at a temperature at which the polymer is present in liquid or dissolved form in the reaction mixture. In general, these temperatures are from 50xc2x0 C. to 300xc2x0 C. The solid does not always have to be separated off completely in step b), since small amounts of solid often do not interfere appreciably in the polymerization in step c). The solid is typically separated off to an extent of from 50% to 100%, preferably from 90% to 99.9%.
The reaction mixture which has been freed of the solid is then polymerized further in step c). The molecular weights of the polymers after step c) are above those after step a) and are from 1000 g/mol to 500,000 g/mol, preferably from 2000 g/mol to 300,000 g/mol and particularly preferably from 3000 g/mol to 100,000 g/mol.
It is also possible to increase the concentration of the polymers prior to step c) by concentrating the clear solution from the sedimentation apparatus by evaporation of solvents and other volatile components. It has been found to be advantageous to concentrate the solution to a polymer content of from 20 to 70 percent by weight, preferably from 30 to 60 percent by weight.
In addition, up to 20 mol percent, preferably from 0.5 to 5 mol percent, based on the amount used in step a), of one or more monomers can be added between steps b) and c) in order to ensure a very precise stoichiometric equivalence between the monomers in the case of a copolymerization, a polycondensation or a polyaddition. Finally, volatile components can be removed from the mixture by passing steam through it in order to remove undesirable constituents.
The separation process of the invention will be described using the separation of undissolved sodium chloride from polyphenylene sulfide (PPS) dissolved in N-methylpyrrolidone as an example, without being restricted thereto.
A suspension of sodium chloride in a homogeneous solution of PPS in NMP is obtained by reacting para-dichlorobenzene (DCB) with sodium sulfide hydrate in NMP at from 180xc2x0 C. to 280xc2x0 C. in accordance with the prior art. In a preferred embodiment of the invention, the reaction is carried out at from 220xc2x0 C. to 250xc2x0 C. in a continuous stirred tank, a cascade of stirred tanks or a combination of stirred tanks and a flow tube reactor. The monomers and the solvent are introduced continuously into the reaction system and the reaction mixture is likewise discharged continuously from the reaction system.
The conversion of the monomers sodium sulfide and DCB in the reaction mixture discharged is above 50%. The reaction mixture comprises, as main constituents, solvent, homogeneously dissolved PPS and insoluble sodium chloride. Secondary constituents which may be present in the solution are unreacted monomers, water and reaction promoters known from the prior art. Known promoters of this type are, inter alia, the acetates or halides of alkali metals or alkaline earth metals, for example lithium chloride or sodium acetate. Under conditions typical for the industrial preparation of PPS, the molar ratio of NMP to sulfide is from 1 to 10, preferably from 2 to 5. The proportion by volume of the sodium chloride in the reaction mixture is V(solid)/V(total)=0.05-0.25, preferably 0.1-0.2.
The reaction mixture is fed to the sedimentation apparatus under the action of gravity, a pressure difference or by means of a pump. It is operated at a temperature of from 180xc2x0 C. to 280xc2x0 C., preferably from 220xc2x0 C. to 250xc2x0 C. The operating pressure of the sedimentation vessel is determined by the vapor pressure of the reaction mixture at the operating temperature. It is from 0 to 15 bar gauge pressure, preferably from 1 to 5 bar gauge pressure.
The separation of the solid from the reaction mixture in step b) is carried out at a reaction conversion based on the aromatic dihalo compound of from 50% to 98%, preferably from 50% to 96%, particularly preferably at from 60% to 94%.
During the reaction in step a), it is usual for chemically bound water of hydration to be liberated. It can be advantageous for the sedimentation in step b) to remove all or some of the water of reaction. If desired, the contents of the reactor can be neutralized or made slightly acidic by addition of acids prior to removal of the solid. Examples of suitable acids are acetic acid, hydrochloric acid and carbon dioxide.
If the mixture to be separated is close to its boiling point, it can be cooled somewhat or diluted with solvents before it enters the sedimentation apparatus in order to avoid formation of bubbles of vapor in the sedimentation apparatus.
The sedimentation residue is advantageously washed with solvents to remove adhering residues of mother liquor. The washing process preferably takes place in the sedimentation apparatus itself. The solid discharged with solvents is dried in order to recover adhering solvent residues. As a result of this separation operation, sodium chloride is obtained as solid and the polymer is obtained as a solution in the overflow from the sedimentation apparatus.
It is also possible to remove all or part of the excess dihalo compound prior to step c). This can be achieved, for example, by feeding the overflow from the sedimentation apparatus to the top of a multistage column which is superposed on a vaporizer. The concentrated polymer solution which has been largely freed of dihalo compound is taken off from the vaporizer. Part of the solvent is vaporized in the vaporizer and the solvent vapor passes in countercurrent through the column, as a result of which compounds which are more volatile than the solvent are removed.
The reaction conditions for the further polymerization in step c) can be varied within wide limits. Thus, the reaction temperatures can be from 180xc2x0 C. to 290xc2x0 C., preferably from 230xc2x0 C. to 270xc2x0 C. The reaction times can be from 10 minutes to 10 hours, preferably from 20 minutes to 2 hours. In the continuation of the polymerization in step c), it is possible, if desired, to employ additional measures which lead, according to the prior art, to achievement of very high molar masses. These include, for example, the addition of promoters. Examples of known promoters of this type are alkali metal salts and alkaline earth metal salts of lower carboxylic acids, in particular sodium acetate. It is likewise possible to add defined amounts of water in step c) in order to carry out the further polymerization in a two-phase reaction system, as described in the prior art. Finally, further additives such as acids, e.g. acetic acid or hydrogen sulfide or carbon dioxide, can be added to adjust the base strength of the system.
Various methods are available for isolating the polymer. The polymer can be separated off by, for example, crystallization and subsequent simple pressure filtration. However, it is also possible to use other methods of separating solids from liquids, for example centrifugation or decantation. The resulting suspension can also be worked up in a depressurization vaporization or by spray drying. Here, solvents and further low molecular weight substances as main constituents are taken off in vapor form and the polymer is obtained as a largely dry solid. A further possibility is evaporation of the polymer solution until a polymer melt is obtained.
The invention further provides an apparatus for separating at least one dissolved polymer from solids using at least one sedimentation apparatus, wherein the solid is separated off by sedimentation and the sedimented solid is washed in countercurrent with a solvent. Very good results are achieved using at least one sedimentation apparatus. The apparatus of the invention is suitable for separating sulfide-containing polymers, in particular polyphenylene sulfide, from solids.
The invention accordingly provides an apparatus for preparing at least one polymer in a solvent, wherein
a) a polymer having a low molecular weight is prepared in solution,
b) the solid which is undissolved in the reaction mixture is separated off in an apparatus comprising at least one sedimentation apparatus and
c) the mixture which has been freed of the solid is polymerized further until a particular molecular weight has been reached.
Very good results are achieved by means of the apparatus, particularly in the case of sulfur-containing polymers, in particular in the case of polyphenylene sulfide.
The process of the invention is illustrated below with the aid of a drawing and examples.